The aluminum industry is reported to consume approximately 7% of all electrical energy used in industry in the U.S. for the production of primary aluminum metal, i.e., metal derived from ore as distinct form recycled aluminum. The current annual domestic capacity for primary metal production is about 5 million tons, which represents a daily energy use of approximately 200 million killowatt-hours. Although the average U.S. specific energy consumption has been reduced about 30% over the past twenty-five years, significant further savings with present technology appear possible only in newly constructed smelters. Unfortunately, the aluminum industry is one of the most capital intensive: replacing the complete U.S. reduction capacity with new low-energy type reduction cells would require an enormous investment. This is economically impractical; a better solution is a means of reducing the energy required to produce aluminum in existing facilities.
The Hall-Heroult smelting process (referred to hereinafter as the Hall Process), in which alumina is dissolved in molten NaF--AlF.sub.3 salt at 940-980 deg. C and electrolytically decomposed with direct current, is universally used to make aluminum metal. A typical aluminum smelter (Hall cell) has an energy efficiency of about 40%. A major portion of the energy consumed in the process can be attributed to the voltage drop between the anode and cathode. The spacing between the anode aND cathode (known as the anode-cathode distance, or ACD) must be maintained at 4-5 cm to prevent electrical shortage between the carbon anode and the molten aluminum pool which serves as the cell cathode. Therefore, if it were possible to replace the turbulent metal cathode with a dimensionally stable cathode, the ACD would be reduced significantly with a concomitant saving in energy.
The potential benefits of using electrically conductive titanium diboride (TiB.sub.2) for this cathode application have been recognized for over twenty-five years. However, past attempts to apply TiB.sub.2 cathodes to full-size reduction cells have been frustrated by short-lived parts. The materials that have been available were susceptible to intergranular penetration by molten aluminum, which eventually resulted in complete physical disintegration. This is caused by intergranular swelling occuring as a result of the reduction of grain boundry oxide impurities by aluminum that penetrates these bodies. This attack may be, in part, caused by the sintering aid necessary for making TiB.sub.2 bodies. Other material properties compounded this problem: TiB.sub.2 is brittle, thermal shock sensitive, and cannot withstand mechanical impact.
As improved TiB.sub.2 materials have been developed, additional studies have been made. Some improvement in longevity of the cathode parts have been demonstrated by Kaiser aluminum and chemical corporation under contract with the U.S. Department of Energy. This work has been reported in Reports DOE/CS/40215-1 and -2. The cathode constructions investigated were components fabricated from hot-pressed TiB.sub.2, and carbon blocks coated (troweled coatings or glued prebaked plates on carbon) with TiB.sub.2. Of these, the coated cathode structures were superior to the hot-pressed bodies; however, the costs of fabrication are high (large quantities of TiB.sub.2 are required) and the performance in molten aluminum was marginal.
Accordingly, it is an object of the present invention to provide a cathode structure for a Hall cell that permits use of a smaller anode-cathode distrance than in conventional Hall cells.
It is another object to provide a titanium diboride cathode structure for use in a Hall cell that has sufficient integrety so as to reduce replacement cycles for use in the cell.
Another object of the present invention is to provide suitable titanium diboride cathode for Hall cells that can be fabricated at reduced cost by reducing to a minimum the requirement for costly TiB.sub.2.
A further object is to provide a cathode structure for Hall cells that has improved fracture toughness and utilizes no sintering aid to give rise to deleterious attack.
These and other objects of the invention will become apparent upon a consideration of the full description hereinafter.